With the increase of unitary power of power generation units, it is becoming more and more common to use power conversion topologies based on multilevel power converters due to the increase of the unitary power of wind turbines. Together with the development of technology, grid codes have become more demanding, and requirements and recommended practices have been established regarding the power quality delivered to the grid.
It is known that power converters, included in power conversion systems used for transforming energy generated from a variable source for connection to the grid, produce output currents and voltages that include harmonic components at the switching frequency (SWF) of the power converters and multiples of those harmonic components. With the objective of limiting those harmonic components below certain maximum values, the installation of filters at the alternating current side such as LC or LCL filters at the output of the power converters 103′ is commonly known in power conversion systems 100′, as shown in FIG. 1 for example, and several solutions have been analyzed related to the design of said type of filters. Some solutions include using an output reactor for the output of each power converter, an RC branch, and a second reactor connected to the grid.
A commonly used design criteria, as disclosed for example in “LCL filter design and performance analysis for Grid Interconnected Systems (IEEE Transactions on Industry Applications VOL 50, No. 2, March/April 2014, pages 1125/1232)”, is to select a filter at the alternating current side of the converter comprising a configuration or topology with a resonant frequency (fres) is far enough from both the switching frequency (fsw) of the power converter and the fundamental frequency of the grid (fg), according to the following equation:10fg<fres<0.5fsw 
Also, a damping resistive element is usually included for attenuating the resonance of the filter.
However, in some applications it is difficult to meet that commonly used design criteria, for example when designing filters for medium-voltage converters for high power applications, because in said applications the switching frequency of the power converter is limited to about 1 kHz due to the junction temperature constraint of the semiconductors of the power converter. Thus, the frequency band between the fundamental frequency of the grid and the switching frequency of the power converter is limited to a little more than one decade, and accordingly, it is a challenge to design the LCL filter design to meet grid requirements for grid connected applications.
Traditional designs are focused on the optimization of the filter parameters and different damping circuits at resonance frequencies, through the selection of a determined impedance value that ensures fulfillment of the grid code power quality requirements. However, these designs might not be optimum from the point of view of efficiency, since depending on the damping impedance required, the power losses at fundamental frequency increase.
In order to avoid the use of inefficient damping circuits, some prior solutions propose damping the harmonics through modulation techniques, as disclosed in “LCL Grid Filter Design of a Multi-Megawatt Medium-Voltage Converter for Offshore Wind Turbine using SHEPWM Modulation (IEEE Trans. Ind. Electron., vol. 31, no. 3, pp. 1993-2001, March 2016)”. However, although these technics are adequate for steady state conditions, they have limitations during transients since the control is not able to damp the resonances fast enough.
In the patent document US20130039105A1 a controllable filter topology at the alternating current side of a power converter is proposed. The filter includes a plurality of capacitors and a single damping resistive element, a rectifier connected between the capacitors and the single damping resistor, and a switching element for disconnecting the damping resistor during start-up of the power conversion system, said damping resistor being connected once the system is under steady state conditions (under normal operation). With said controllable filter topology and the control method proposed in said patent document, the power factor of the conversion system during the start-up is modified, at the same time that the filter maintains its filtering ability during steady state conditions (or normal operation). However, this solution is not focused and does not provide a solution for the problems that could arise when the system has to cope with grid transients (transient state), such as fault ride through or overvoltage events, for example. During a transient state, the non-dampened resonance frequency of the filter is excited, the control band width does not allow controlling the transient, and the reactive current injection time requirements cannot be fulfilled. This could imply the loss of the control of the currents until the transitory response is mitigated due to the parasitic impedances of the system.